1. Field of the Invention
The present invention generally relates to semiconductor devices and method of forming the same, and in particular, the present invention relates to the microlens structure of image sensors and to methods of forming the same.
A claim of priority under 35 U.S.C. § 119 is made to Korean Patent Application No. 2004-98665, filed on Nov. 29, 2004, the entire contents of which are hereby incorporated by reference.
2. Description of the Related Art
The digital camera is a popular example of a device which utilizes image sensor having an optical microlens and a light receiving element. Light incident on the microlens is focused towards the light receiving element which transforms the light into an electrical signal capable of image processing.
The image sensor is usually composed of a large two-dimensional pixel array, where each pixel includes an optical microlens and a light receiving element. In addition, transmission and signally devices are provided to read out the images of the image sensor. Most image sensors can be classified as either charge-coupled-device type image sensors (CCD images sensor) or complementary metal-oxide-semiconductor (CMOS) image sensors (CIS).
In both CIS and CCD image sensor, as the integration density continues to increase, the spacing between adjacent pixels is becoming narrower. As a result, light incident at a target pixel tends to outflow toward an adjacent pixel. The problem is explained below in more detail with reference to FIG. 1.
Referring to FIG. 1, the conventional image sensor includes a light shielding layer 104 located within interlayer insulation films 105a and 105b. The light shielding layer 104 is intended to prevent light from being incident on a region 103 external the light receiving devices 102a, 102b, and 102c. Color filters 106a, 106b, and 106c are arranged over the light shielding layer 104. On the color filters 106a˜106c, a protection film 107 having a flattened top surface is formed for planarization and improving optical transmittance. Also, microlenses 108a, 108b, and 108c are arranged on the protection film 107 to concentrate light thereon.
In general, the light receiving devices 102a˜102c are formed of photo gates or photodiodes. The light shielding layer 104 is formed of metal. The color filters 106a˜106c are made of dyed photoresist materials. The microlenses 108a˜108c are mostly formed with using polymer resins, and the protection film 107 is usually made of silicon oxide that has a refractive index similar to that of the microlens.
In such a conventional image sensor, one side (upper surface) of the microlens (e.g., one of 108a˜108c) is shaped with a convex curvature, whereas the other side (lower surface that faces the protection film 107) is flat. In other words, the conventional microlens is configured with a unilateral convex top surface. The efractive index of the protection film 107 is substantially equal to that of the microlenses 108a˜108c. Thus, as shown in FIG. 1, a focal distance dF1 of the microlens is shorter than a distance dA1 from the microlens to the light receiving device. It means that a focus F of the microlens is settled on a position spaced well away from the light receiving device. As a result, the focusing efficiency is degraded and light rays are likely to be incident on adjacent light receiving devices.